Igniter for solid propellant



March 16, 1965 T. E. LANDRY, JR., ETAL IGNITER FOR SOLID PROPELLANT Filed Dec. 26, 1961 INVENTORS 7Z/E000RE E LANDRY JR, DONALD 1M HANSON /MM 5 W AGENT United States Patent Ofiice 3,173,370 Patented Mar. 16, 1965 tion of California Filed Dec. 26, 11.961, Ser. No. 162,016 2 Claims. (Cl. 102-70) The present invention relates to igniter assemblies for solid propellant rocket motors, and its primary object is to provide a new and an improved igniter in the form of a self-supporting sheet or film of plastic or like material, which can be bent or curled to conform to the configuration of the propellant grain port and inserted endwise therein.

Another important object of the invention is to provide an igniter assembly in the form of a sheet, as described above, wherein a mild detonating fuse or other cord-like detonating means may be affixed in a predetermined pattern to the sheet so that when the igniter assembly has been placed in the propellant grain port and the fuse initiated by a primer, a shock wave is propagated uniformly over the entire burning surface of the propellant grain. One of the features of the invention is that the cord-like detonating means is firmly attached to the sheet, and therefore retains its pattern while being inserted endwise into the restricted confines of the propellant grain port.

Another object of the invention is to provide an igniter assembly of the class described, in which means is provided for securing a plurality of capsules, or measured quantities, of pyrotechnic composition to the said sheet at more or less uniform intervals from the said cord-like detonating means along the length thereof, which are ignited almost simultaneously by the shock wave of the cordlike fuse, and burn hotly to ensure ignition of the propellant grain.

A further object of the invention is to provide an igniter assembly of the class described, in which the supporting sheet or film is formed of combustible material that is ignited by the pyrotechnic composition and is consumed almost instantaneously, so that there is virtually no debris to be blown out through the exhaust nozzle.

Another object of the invention is to provide an igniter assembly of the class described, having the following desirable characteristics: (1) versatility to accommodate a wide range of requirements due to ignition characteristics of different propellants, operating environments, and larger rocket motor cavities; (2) low brisance ignition; (3) minimum ignition delay or lag; (4) structural rigidity to permit insertion endwise into the long, narrow, blind cavities of large solid propellant rocket motors; (5) minimum ignition debris to be expelled through the nozzle; and (6) distribution of ignition material over the entire burning area of the propellant grain, as opposed to the concentration of the ignition material in a large central charge, or in a linear tube charge.

These objects are achieved in the preferred form of our invention by providing a thin sheet of plastic or the like, such as cellulose nitrate, for example, which is molded or otherwise shaped to provide a plurality of closely spaced bosses that define channels or grooves into which a strand type detonating material can be inserted. This strand type detonating material, sometimes referred to as detonating fuse, is a small diameter flexible metal tubing containing a detonating composition such as PETN, having a propagation velocity of the order of 3000 to 5000 meters per second. Additional ignition material may be packed into certain of the cavities of the sheet, if required, to provide continuing heat and pressure energy for the ignition process until the motor is operating satisfactorily.

The foregoing and other objects and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment thereof, reference being had to the accompanying drawings, wherein:

FIGURE 1 represents schematically a typical solid propellant rocket motor, including an igniter assembly embodying the present invention;

FIGURE 2 is a cross-sectional view taken through the same, on the line 22 of FIGURE 1,

FIGURE 3 is an enlarged perspective view of a portion of the igniter sheet assembly of the present invention, showing the manner in which the detonating fuse is inserted down between the bosses, or protuberances, on the sheet; and

FIGURE 4 is a sectional view, taken at 4-4 in FIG- URE 3 showing several different forms of ignition material packed into the pockets of the sheet.

The basic concept of the present invention is to provide a relatively stifi, self-supporting sheet or film that can be curled or bent to conform to the surface of a propellant grain port and inserted endwise therein, and which is provided with means to initiate combustion of the propellant grain almost instantaneously over the entire surface area thereof. The port shape or size, the configuration of the solid propellant grain, are immaterial. It is contemplated that the ignition sheet or film be curled, bent, or otherwise formed to allow it to be inserted endwise into the port and lie against or closely adjacent the burning surface over the entire area thereof, so that ignition of the sheet will initiate combustion of the propellant.

Broadly, it is intended that a plastic sheet or film, impregnated throughout with ignition material, or made of combustible material such as cellulose nitrate, and having a detonation fuse incorporated therein, be formed to fit the port of the propellant grain so that ignition is instantaneous over the entire inner area. It is also intended, in a more limited sense, to provide an igniter sheet or film having a plurality of protruberances or bosses molded therein, which form grooves or channels, into which detonating fuse may be inserted and held in place, the said bosses also providing pockets into which pyrotechnic material may be packed. Detonation of the fuse is virtually instantaneous, causing instantaneous ignition of the pyrotechnic material, which results in instantaneous ignition of the propellant grain over the entire burning area.

It is to be understood that the shape and size of the grain port areas of solid propellant rocket motors are many, that the drawings merely represent a single disclosure setting forth the novel structure and arrangement of elements. The type of supporting sheet, whether partially of plastic, or entirely of ignition material, is a matter of choice. The manner of indenting or embossing the plastic sheet to receive the ignition material could be accomplished in many ways. The cord-like detonating fuse may be attached to the supporting sheet in any way.

In the drawings, the reference numeral 5 designates a rocket motor casing which is tubular in configuration and is closed at one end. A-closure 6 is removably secured to the other end of the casing 5, and is provided with an integral nozzle 7, through which the combustion products are exhausted at high velocity, to produce reaction thrust. Loaded within the casing 5 is a grain 8 of solid propellant, having a narrow, blind, annular propellant grain port 9 concentric with the longitudinal axis of the casing. While the grain port 9 of the drawings is annular in configuration, this is merely illustrative, as there are many other cross-sectional shapes in common use, and the present invention may be used with equal advantage in any of these various port configurations.

Disposed within the propellant grain port 9 is the sheet igniter assembly of the present invention, which is indicated in its entirety by the reference numeral 16). In the illustrated embodiment, the igniter assembly is cylindrical, but it will be understood that this is merely because the port 9 is shown as a cylindrical cavity. Whatever the cross-sectional shape of the port, the sheet igniter assembly of the present invention would be bent or curled to give it the same shape, so that the igniter assembly could be inserted endwise into the port and lie closely against the exposed surfaces thereof.

The igniter 10 consists of a thin, fiat sheet 12, of cellulose nitrate or other like plastic, which is structurally embossed to form a plurality of bosses, or protuberances 14 that are preferably arranged in parallel rows, with the bosses of each row offset to lie between the bosses of the adjacent rows, and the rows of bosses being spaced apart to provide narrow grooves, or passageways 16, when viewed from one edge of the sheet 12, as in FIGURE 4. The bosses 14 are preferably formed with cylindrical side walls 18 and fiat, circular tops 19, so that the channels 16 defined between adjacent rows of bosses have parallel sides, and the depth of the channels is preferably greater than the width thereof, as seen from an edge of the sheet 12.

Pressed down into the channels 16 is a strand-like detonating fuse 26, which extends back and forth over the surface of the sheet 12 in a predetermined pattern such that adjacent lengths of the fuse are spaced apart more or less uniformly from one another at a distance slightly greater than the effective radial reach of the shock wave propagated by the fuse. In this way, the fuse is prevented from being disrupted by its own shock wave in advance of the longitudinal propagation of the detonation.

The detonating fuse 20 is preferably of the type sold by E. I. duPont de Nemours and Company under the trademarks Pyrocore and MDF, and consists of a flexible tubular metal sheath 22, containing a central core 24 of detonating composition such as PETN. The fuse 2t) resembles solder wire in appearance, and in flexibility, and can readily be pressed down into the channels 16, where it is held in place by friction. The outside diameter of the fuse 20 is preferably slightly greater than the width dimension of the channel 16, so that the fuse is bent into a slightly sinuous line in passing between the offset bosses of adjacent rows of bosses, and is thereby held snugly in place. The fuse 20 may be additionally secured in place on the sheet 12 by means of dabs of potting compound 26 placed on top of the fuse between two adjacent bosses 14 'on opposite sides of the channel 16, which adhere to the associated bosses and to the fuse, and thereby cement the fuse to the sheet. The ends of the fuse 20 protrude from the edge of the sheet 12, and are attached to an electrical detonating primer 30, having wires 32 which pass out through the nozzle 7 and are connected to an electrical firing circuit (not shown). When the primer is initiated, the composition 24 is detonated, and the detonation is propagated along the length of the fuse at a velocity of 3000 to 5000 meters per second. Thus, detonation of the entire length of fuse 20 is virtually instantaneous.

While the detonation shock wave of the fuse 20 may be sutficient in some cases to ignite the propellant grain 8, it is usually desirable to augment the fuse detonation by providing continuing heat and pressure energy for the ignition process until the motor is operating satisfactorily. Part of the continuing heat and pressure energy may be provided by the sheet 12, itself, which may be made of cellulose nitrate or other combustible material, either with or without oxidizer, depending upon the combustibility of the material. In this case, the sheet 12 would be ignited instantaneously over its entire area by the fuse 20, and combustion of the sheet would provide the additional heat and pressure energy needed to ensure complete ignition of the propellant grain 8 over the entire burning area of the port 9.

A more effective source of continuing heat and pressure energy may be obtained by the use of pyrotechnic material which is incorporated into the sheet 12, or aflixed thereto, as in the embodiment illustrated herein. For this purpose, pyrotechnic material in the form of powder, cast ignition material, ignition Wire, or pellets, may be loaded into certain of the cavities 34 defined by the concave sides of the bosses 14. The cavities 34 that are loaded with pyrotechnic material are preferably those located immediately adjacent the fuse 26, and preferably alternating from one side of the fuse to the other. The cavities loaded with pyrotechnic material may be spaced apart lengthwise of the fuse 2t) so that the loaded cavities are interspersed more or less uniformly over the entire area of the sheet 12, and the distance between adjacent loaded cavities is such that their effective radii overlap.

FiGURE 4 shows two different forms of pyrotechnic material loaded within cavities 34, one of which is designated 36 and the other 36'. The material shown at 36 may be powder, or it may be in the form of a solid mass cast in the cavity while in a fluid or semi-fluid state and then hardened in place. The pyrotechnic material 36 may be any composition which is readily ignitable and burns to produce much heat, such as finely divided metal and a solid oxidizing agent, usually a chlorate or perchlorate salt. Typical pyrotechnic compositions are aluminum, or boron, or magnesium mixed with potassium perchlorate or potassium chlorate. For the cast-in-place pyrotechnic material, a liquid resin vehicle, such as polyester or epoxy resin with suitable catalyst, may be added to the dr powdered ingredients to form a paste or slurry which may be poured, extruded, or packed into the cavities 34. The liquid vehicle then hardens, leaving the pyrotechnic material in the form of a hard solid. The material 36 may be covered by a plastic disc 38 which is cemented or bonded around its edges to the marginal edge of the cavity.

The pyrotechnic material shown at 36' is in the form of a pellet, or tablet, which is preformed with a suitable binder, such as polyester or epoxy resin. The tablet 36' has the advantage of convenience in handling, as the sheet 12 may be loaded by merely dropping one tablet into each of the selected cavities 34. The tablets 36' are retained in place by a plastic disc 38, which covers the cavity and is cemented or bonded around its edges to the sheet 12.

In addition to the powder, cast-in-place solid, and tablet forms just described, the pyrotechnic material might also be in the form of a solid or semi-solid foam, or it might be in the form of a fine metal wire with a measured quantity of dry oxidizer packed loosely with the wire in the cavity 34.

The present invention also contemplates that the sheet 12 might be made of some material other than plastic, such as, for example, a woven fabric of silk, rayon or the like, which has been stiffened somewhat to give it structural rigidity so that the fabric, with attached fuse 20 and with or Without quantities of pyrotechnic material interspersed over the area of the fabric, may be bent or curled to be inserted endwise into the narrow, blind cavity of the propellant port 9. The fabric may be stilfened by application of cellulose nitrate lacquer, which also renders it more inflammable, and additional oxidizer or pyrotechnic composition may be admixed with the lacquer or sprinkled over the surface of the fabric while the lacquer is still wet, so that the powdered or granular composition will adhere to the fabric. The fuse 20 may be attached to the surface of the fabric by means of staples or dabs of cement.

The operation of the invention is believed to be selfevident from the foregoing description. To prepare an igniter assembly for a rocket motor such as the one illustrated in FIGURE 1, a sheet of embossed plastic 12 may be cut to size from a supply roll, with its width dimension equal to the circumference of the port 9, and its length equal to the depth of the port. The fuse 20 is then laid out in the desired back-and-forth pattern over the entire area of the flat sheet 12, and it subsequently pressed down into the grooves, or channels 16, and secured by dabs of potting compound 26 if desired. Certain of the cavities 34 are then filled with pyrotechnic composition, which may be sealed in place by plastic discs 38, and these loaded cavities are preferably distributed uniformly over the surface of the sheet 12 in such number and at such spacing with respect to one another, that the instantaneous ignition of the pyrotechnic charges 36 by the fuse 20 causes the entire burning surface of the port 9 to be ignited instantaneously.

The assembled igniter 10 is now bent or curled to the same cross-sectional configuration as the port 9, and is inserted endwise into the port so that it reaches to the very bottom thereof. The protruding ends of the fuse 20 are connected to an electrical igniter 30, and the wires 32 of the latter are connected to the firing circuit. In the rocket motor of FIGURE 1, the end closure 6 and nozzle 7 are removable from the case 5 to provide convenient access to the port 9. This may not always be necessary, however, as the port 9 is sometimes of smaller diameter than the inside diameter of the nozzle throat, and in this case the igniter assembly would be inserted into the port 9 through the nozzle.

While we have shown and described in considerable detail what we believe to be the preferred embodiment of our invention, it will be understood by those skilled in the art that various changes may be made in the several parts without departing from the broad scope of the invention as defined in the following claims.

We claim:

1. An igniting device for a solid propellant grain comprising,

a self-supporting sheet of plastic material having a plurality of protuberances integral therewith,

a strand-like detonating fuse of high detonation velocity extending back and forth between adjacent pairs of said protuberances to thereby cover substantially the entire area of said sheet,

a plurality of charges of pyrotechnic material enclosed within at least a portion of said protuberances at spaced intervals along and closely adjacent said fuse, said pyrotechnic material being ignitable by said fuse,

said sheet being capable of being bent and inserted into a port in a propellant grain.

2. In combination with a solid propellant grain having a narrow port extending within said propellant grain in a predetermined configuration, and wherein it is desired to simultaneously, instantaneously and uniformly ignite all of the burning surfaces of the propellant grain, the exposed surface of the propellant grain within said grain port constituting the burning surface,

an igniting device disposed within said port comprising,

a self-supporting sheet of ignitable flexible material capable of being bent to conform to said grain port and capable of being inserted endwise therein,

a strand-like detonating fuse of high detonation velocity fixed to said sheet in a pattern providing uniform dispersal of said fuse over substantially the entire area of said sheet,

a plurality of hollow bosses, each containing a charge of pyrotechnic material, fixed to said sheet at intervals along said fuse and dispersed substantially uniformly over the area of said sheet and so arranged as to provide attaching means for said fuse,

said charges being ignitable by the detonation of said fuse for providing continuing heat and pressure energy to ensure ignition of said propellant grain.

References Cited in the file of this patent UNITED STATES PATENTS 2,743,580 Loeb May 1, 1956 2,995,088 Asplund Aug. 8, 1961 3,014,425 Turnbull et a1. Dec. 26, 1961 

1. AN IGNITING DEVICE FOR A SOLID PROPELLANT GRAIN COMPRISING, A SELF-SUPPORTING SHEET OF PLASTIC MATERIAL HAVING A PLURALITY OF PROTUBERANCES INTEGRAL THEREWITH, A STAND-LIKE DETONATING FUSE OF HIGH DETONATION VELOCITY EXTENDING BACK AND FORTH BETWEEN ADJACENT PAIRS OF SAID PROTUBERANCES TO THEREBY COVER SUBSTANTIALLY THE ENTIRE AREA OF SAID SHEET, A PLURALITY OF CHARGES OF PYROTECHNIC MATERIAL ENCLOSED WITHIN AT LEAST A PORTION OF SAID PROTUBERANCES AT SPACED INTERVALS ALONG AND CLOSELY ADJACENT SAID FUSE, SAID PYROTECHNIC MATERIAL BEING IGNITABLE BY SAID FUSE, SAID SHEET BEING CAPABLE OF BEING BENT AND INSERTED INTO A PORT IN A PROPELLANT GRAIN. 